Arkell.1956.Jurassic..

SWABIAN AND FRANCONIAN ALB IO7
general shallowing and shrinking of the Upper Jurassic seas in central
Europe. Some of the sponge reefs have a spectacular expression in the
landscape, standing out from the escarpment edge as white crags, too steep
to give foothold to the forest. Sometimes a castle is perched on the summit.
The Black Forest and the Rhenish slate mountains probably were not
the nearest land to the Swabian trough of deposition; another shore
probably lay to the south-east, not far from the present line of the Danube.
This was the Vindelician landmass (so called by Giimbel after the Roman
province of Vindelicium, the capital of which was at Augsburg), also
sometimes called the Alemannian landmass, which existed during the
Lias and began to be submerged in the Toarcian and early Lower Bajocian
but persisted at least as a string of islands to the Callovian or Oxfordian,
when it broke up and sank below the sea (Frank, 1937a, p. 76). It was a
south-westerly prolongation of the Bohemian Forest and originally linked
the Bohemian and Black Forest massifs. Direct proof has been afforded by
a boring south-west of Augsburg, which showed Lower Bajocian Opalinum
Zone clays resting on continental Trias (93 m.) and that on gneiss (Roll,
1952). The Bohemian and Vindelician massifs were no doubt the chief
suppliers of at least the sandy sediment of the Swabian and Franconian
Middle Jurassic (Schmidtill, 1925, p. 79; Bozenhardt, 1936, p. 78), but it
has not been possible to establish mineralogically the source of the sand
grains (Aldinger, 1953).
To the south-east, a gulf from the Franconian Jurassic sea extended
from Regensburg down the Danube valley towards Passau (von Ammon,
1875; Pompeckj, 1901; Wanderer, 1906). When the Vindelician land broke
up a direct connexion was established this way with the Cracow area of
Poland, by a sea skirting south of the Bohemian Forest. Close resemblance
between the Stephanoceratids of Franconia and the eastern Alps indicates
a seaway across the Vindelician barrier hereabouts as early as the Middle
Bajocian (Schmidtill & Krumbeck, 1938, p. 322).
Towards the north, on the contrary, and in the west (Black Forest)
uplift and retreat of the sea took place about the same time as subsidence
in the south and east. In Lower and Middle Jurassic times the Alb trough
connected through a strait between the Rhenish and Bohemian massifs
with the North German sea, but with the advent of the Upper Jurassic
this strait appears to have been closed by uplift of the Thuringian Forest
(Pompeckj, 1908). The Black Forest also rose above sea in the early
Upper Jurassic if not in the Bajocian (see p. 39).
In Franconia at the time of deposition of the celebrated lithographic
'slates' of Solnhofen and Eichstatt the sea is thought to have contracted
to a small lagoon, surrounded by Upper Jurassic rocks already raised into
low-lying land (Roll in Dorn & others, 1935, p. 669). In and around the
shallow lagoon, in tropical heat, thrived a rich mixed land and sea fauna—
pterodactyls, archaeopteryx, dragon-flies, jellyfish, and a great variety of
other animals—miraculously preserved in the lithographic limestone.
(For attractive illustrated descriptions see Walther, 1904, and Abel, 1922).
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